1. Field of the Invention
The present invention relates to a coating process for applying plastic coatings on to planar substrates by use of a coating head having a distributing channel extending over the application width and which is provided with a supply channel issuing into the distributing channel and connected to a duct leading to a feed pump and a coating gap connected to the distributing channel.
2. Description of the Related Art
A coating head of this type is described in German Pat. No. 26 14 596. There, the coating gap connected to the distributing channel has a relatively small thickness and a relatively great length. Thus, the coating material is exposed to a high flow resistance in the coating gap. As a result, the static pressure of the coating material is the same at all points within the distributing channel, so that the coating material flows out of the entire width of the supply gap at the same speed and consequently dead areas in the system are avoided. As a result of this construction such a coating head is particularly suitable for producing homogeneous coatings and films made from highly transparent plastics, e.g., multicomponent reaction mixtures whose reaction begins within the coating head and in which dead areas in the system lead to striations in the transparent coating.
Preferred uses for such coating heads are as scraping coating heads in the production of coatings and films of highly transparent polyurethanes which can be used as intermediate coatings and/or splinterproof protection coatings for multilayer windscreens. The coating substrate for such coatings can either be the actual substrates to be provided with the coating, or other planar substrates from which, following the curing or complete hardening of the coating material, the cured coating is removed as a foil.
The upper edge of motor vehicle windscreens is often provided with a colored filter strip which reduces solar radiation. Generally this colored filter strip is located in the thermoplastic sheet interconnecting the two individual glass plates of a laminated glass plate. It can be produced by coloring the sheet or by a coextrusion of a colored plastic material and a colorless material. Between the colored strip and the colorless area there is provided a transition area, in which the transparency continuously changes over a distance of one to several centimeters, so that to a certain extent there is a gradual change to the filtering characteristics in this transition area. Government regulations require such a continuous transition area.
Whereas sheets or films or foils of thermoplastic material with a filter strip and a continuous transition area between the highly transparent area and the filter strip can be produced by known extruders, the production of coatings or films with a filter strip of reaction mixtures using a coating head has not hitherto been possible.